Maintaining the integrity of the anatomic components of the eye facilitates the delicate manipulations, performed within small areas, of ophthalmological surgery.
One component which may be controlled is the anterior chamber of the eye. As shown in FIG. 1, the anterior chamber is located between the cornea and the iris. Just posterior to the iris is the lens, which is interposed between the anterior chamber and the larger vitreous chamber, filled with vitreous humor. Maintaining the structural integrity of the anterior chamber minimizes the risk that the endothelium and/or the iris will be damaged during surgery. The introduction of viscoelastic materials, such as sodium hyaluronate, chondroitin sulfate, hydroxypropyl methylcellulose, and methylcellulose, into the anterior chamber prevents the chamber from collapsing during surgery.
Another component that may be controlled is pupil size. During cataract surgery, it is desirable that the pupil is dilated, so that access to the lens is simplified and the insertion of a posterior chamber implant is facilitated. A variety of mydriatic drugs, such as atropine (a cholinergic blocker), phenylephrine (an adrenergic stimulator), and prostaglandin inhibitors have been used in this regard, and have hitherto predominantly been administered via external application.
Conversely, in refractive implants and secondary aphakic implants, a smaller ("miotic") pupil is desirable, in order to reduce iris trauma, avoid anterior synechias, prevent iris tucking, and facilitate appropriate positioning of the implant. Externally applied pilocarpine and carbachol (cholinergic stimulators), and physostigmine, demecarium bromide, echothiophate iodide, and isofluorphate (cholinesterase inhibitors) have been used for this purpose.
During surgery, however, and in the open eye, the efficacy of topical medication is reduced. Dilution and runoff preclude a continued high dose of effective medication. Even the direct introduction of miotic agents such as acetylcholine chloride or carbachol do not provide long term effects and often require frequent repeated administration into the open eye.
Previous attempts to achieve long-term maintenance of effective drug levels have employed sustained drug delivery technology, using systemic or transdermal administration, or the positioning of a bioerodible drug delivery device external to the eye. Such methods have been used primarily to control intraocular pressure in glaucoma patients. However, prior to the present invention, no method has been devised which combines sustained mydriatic or miotic drug delivery with maintenance of the structural integrity of the anterior chamber.